Explosives found inside a modified printer ink cartridge on board a cargo plane at East Midlands Airport in the United Kingdom were primed to detonate in mid-air.
The device was active when counter-terror police swooped on the aircraft early on Friday.
Yemen-based terrorists had built the bomb to go off in British air space, just like the Lockerbie atrocity of 1988 which killed 270 people.
UK Home Secretary Theresa May yesterday said: “I can confirm that the device was viable and could have exploded. The target may have been an aircraft and had it it detonated, the aircraft could have been brought down.”
Security chiefs initially believed the bomb, which was to be activated via a timer, was destined for a synagogue in the United States.
But Mrs May said: “We do not believe that the perpetrators of the attack would have known the location of the device when it was planned to explode.”
Speaking at Chequers, David Cameron said: “We believe the device was designed to go off on the aeroplane.
“We cannot be sure about the timing when that was meant to take place.”
Had the plane exploded and crashed on to Nottingham, Leicester or Derby, all within a 10-mile radius of the airport, hundreds of lives would have been lost.
The alert was triggered by intelligence from a unit of GCHQ surveillance experts stationed in Afghanistan, the Sunday Express can reveal. Operating from a converted shipping container in Helmand, the team picked up the words “A wedding gift is being delivered”.
The phrase is an Al Qaeda code meaning a bomb is in transit.
With the help of Saudi agents, GCHQ alerted MI6, which raised the alarm in London and Washington.
Counter-terror police intercepted the United Parcel Service plane bound from Yemen to Chicago when it stopped to refuel at East Midlands Airport at 3am on Friday.
The suspect package was found with wires protruding from it and connected to mobile phone components. A similar device was found on board another plane in Dubai.
Yesterday security officials in Yemen announced that investigators had seized and examined 26 other suspect packages in the capital, San’a.
It was also revealed last night that a woman had been arrested in San’a on suspicion of sending explosive packages on cargo planes to the US.
A Yemeni security source said security forces traced the woman through a telephone number she left with a cargo company. Her lawyer, Abdel Rahman Burman, told reporters: “Her acquaintances tell me that she is a quiet student and there was no knowledge of her having involvement in any religious or political groups.”
Authorities also arrested UPS and FedEx workers and were interrogating them last night, along with Muslim employees at San’a Airport.
The East Midlands package is understood to have contained PETN, the explosive chemical used in last year’s failed Christmas Day attack on a Detroit-bound airliner by syringe bomber Umar Farouk Abdulmutallab.
A source close to the investigation said: “The clock on the device was counting down and it was still live as it was discovered on the plane.”
PETN is a colourless, odourless liquid that was probably formed into tiny crystals and poured into the ink cartridge
Explosives expert Mike Coldrick said just an egg-sized amount of PETN held at arm’s length was enough to “blow your head off”.
He added: “It doesn’t take a lot of explosive to bring down an aircraft.”
Abdulmutallab, a Nigerian who studied at University College London, was trained in Yemen.
The country is rapidly becoming a hotbed of Islamic terrorism as Osama Bin Laden’s men seek refuge from US-led forces in Afghanistan and Pakistan. A group calling itself “Al Qaeda in the Arabian Peninsula” is said to be behind the ink cartridge bomb.
SAS troops have now been sent to the Haraz Mountains surrounding San’a to hunt the terrorists responsible for sending the devices. They are being backed by a small team of MI6 agents.
All flights to and from Yemen remained grounded yesterday. Both FedEx and UPS offices in San’a are understood to have stopped accepting shipping to the US.
All unaccompanied air freight from Yemen has been prevented from moving into or through Britain.